1. Field of Invention
The present invention relates to a circuit for zero offset auto-calibration and the method thereof. More particularly, the present invention relates to a circuit for zero offset auto-calibration and the method thereof suitable for an analog-to-digital converting circuit for video signals.
2. Description of the Prior Art
In recent years, digital circuits/devices dealing with video signal always include a analog-to-digital converter (ADC). But the ADC usually has zero offset problem. The so-called zero offset means that there is a DC offset on the signal converting path of ADC, and makes the output not zero under an input of blanking signal. In such a case, the zero offset must be compensated to correct the output.
The attempts to solve the problem have been made before with unsatisfactory results. In the U.S. Pat. No. 5,874,909, a circuit of analog-to-digital converting with a function of auto compensating errors was presented, whereby a calibration digital code, based on digital signal processing, was fed back and converted into an analog signal via a digital-to-analog converter (DAC) to compensate the input signal. As the shortcoming with the means, an additional digital-to-analog converter (DAC) is needed. Besides, the resolution of DAC needs to be enhanced in order to increase the compensation accuracy.
In the U.S. Pat. No. 6,353,405, another kind of analog-to-digital video converter was presented based on a similar principle. A DAC is required as a calibration signal generator, and therefore the same shortcoming to require an additional DAC and expanded area still exists along with an increasing compensation resolution. Besides, a register is also required to hold calibration data.
Thus it can be seen that a better technology needs to be developed to solve the zero offset problem without the shortcomings of the prior art.